The present invention relates to a sintering process for liquid phase sintering of powder metallurgical parts, and the like, to close tolerances without warpage.
U.S. Pat. No. 4,431,605 describes a method for densifying previously sintered parts of powdered materials, etc. The parts may be sintered in either vacuum or hydrogen and may be cooled in a similar atmosphere. The parts are then reheated and the sintering vessel is pressurized to densify the parts.
AT 314212 discloses a method of sintering powder metallurgical parts according to which high pressure is applied after the eutectic temperature of the binder phase has been reached.
Complete densification is possible also without previous sintering. This process is advantageous with respect to dimensional tolerances and warpage of the sintered parts and it also necessitates much less time than the conventional two-step process.
When sintering powder metallurgical parts, the liquid phase, as soon as it is formed during the first heating, wets the solid particles rapidly and forms isolated pores within the structure which then is characterized by closed porosity. It has been found that a certain shrinkage occurs already during this heating. However, the remaining porosity and flaws still make the mechanical properties after cooling inferior compared to fully dense materials. In normal low-pressure sintering, final densification needs a long time for complete pore elimination, which occurs through vacancy diffusion and annihilation.